Recently, secondary batteries, which can be charged and discharged, have been widely used as an energy source or an auxiliary power unit for wireless mobile devices. In addition, secondary batteries have also attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-in REV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Secondary batteries are generally formed in the shape of a rectangular parallelepiped. According to the diversity in design of devices using such secondary batteries, however, there is an increasing necessity for secondary batteries having various shapes.
For a smartphone, which is an example of devices requiring secondary batteries having various shapes, sides or corners of the smartphone may be curved to improve grip. In a case in which the secondary batteries formed in the shape of the rectangular parallelepiped are mounted in a device designed so as to have such curved portions, however, utilization of the inner space of the device may be limited.
That is, the curved portions of the device have dead spaces, in which the secondary batteries formed in the shape of the rectangular parallelepiped cannot be mounted. Ultimately, such dead spaces lower the capacity of the device per unit volume.
A secondary battery is generally manufactured to have a structure in which an electrode assembly is contained in a battery case together with an electrolyte. Based on methods of manufacturing the electrode assembly, the electrode assembly may be classified as a stacked type electrode assembly, a folded type electrode assembly, or a stacked and folded type electrode assembly.
In the stacked type electrode assembly or the stacked and folded type electrode assembly, cathodes and anodes are sequentially stacked in a state in which separators are disposed respectively between the cathodes and the anodes. In order to manufacture the electrode assembly, it is necessary to manufacture the cathodes and the anodes first.
In order to manufacture unit electrode plates, such as anodes and cathodes, it is necessary to notch a long electrode sheet having an electrode active material applied to one major surface or opposite major surfaces thereof at intervals corresponding to the width of each of the unit electrode plates. In the notching process, a press is generally used to notch portions of the electrode sheet. After the electrode sheet is notched, the electrode sheet is cut at intervals corresponding to the width of each of the unit electrode plates.
The above process will be described with reference to FIG. 1A. In the notching process, electrode tabs 20 and first notch portions 30 are formed at the upper side of an electrode sheet 10 such that the first notch portions 30 are arranged at intervals corresponding to the width of a unit electrode plate. Second notch portions 40 are formed at the lower side of the electrode sheet 10 such that the second notch portions 40 are arranged at the intervals corresponding to the width of the unit electrode plate. A left side 41 of each of the second notch portions 40 is formed in an arc shape, by which one corner of the unit electrode plate is rounded.
In the cutting process, the electrode sheet 10 is cut along a virtual cutting line 50 interconnecting each of the first notch portions 30 and each of the second notch portions 40 at the intervals corresponding to the width of the unit electrode plate. However, the electrode sheet 10 is frequently cut at a position deviating from the cutting line 50 to the left or the right by a predetermined width W1 due to a cutting tolerance.
FIG. 1B is a typical view showing a unit electrode plate 60 cut with the cutting tolerance. Referring to FIG. 1B, a protrusion 70 is formed at the lower end of a right side of the unit electrode plate 60. A contour line interconnecting angular points of the unit electrode plate may take a quadrangular shape due to the protrusion 70, which does not conform with a design purpose to manufacture an electrode plate having a round corner. As a result, it may be not possible to manufacture an electrode plate having a round corner. Consequently, there is a high necessity to develop an electrode sheet having novel notch portions to solve the above problems.